In the mid-1800s, biomass, principally woody biomass, supplied over 90% of U.S. energy and fuel needs. Thereafter, biomass energy usage began to decrease as fossil fuels became the preferred energy resources. Today, the world's energy markets rely heavily on fossil fuels, coal, petroleum crude oil, and natural gas as sources of energy. Since millions of years are required to form fossil fuels in the earth, their reserves are finite and subject to depletion as they are consumed. The only other naturally-occurring, energy-containing carbon resource known that is large enough to be used as a substitute for fossil fuels is biomass. Biomass is herein defined as all nonfossil organic materials that have an intrinsic chemical energy content. They include all water and land-based vegetation and trees, or virgin biomass, as well as all waste biomass such as municipal solid waste (MSW), municipal biosolids (sewage) and animal wastes (manures), forestry and agricultural residues, and certain types of industrial wastes. It is understood that today's wastes consist of a mixture of materials derived from fossil fuels and non-fossil organic materials.
Unlike fossil fuels, biomass is renewable in the sense that only a short period of time is needed to replace what is used as an energy resource. Some analysts now believe that the end of the Fossil Fuel Era is in sight because depletion of reserves is expected to start before the middle of the 21st century, probably first with natural gas. This eventuality and the adverse impact of fossil fuel usage on the environment are expected to be the driving forces that stimulate the transformation of biomass into one of the dominant energy resources.
Under ordinary circumstances, virgin biomass is harvested for feed, food, fiber, and materials of construction or is left in the growth areas where natural decomposition occurs. The decomposing biomass or the waste products from the harvesting and processing of biomass, if disposed of on or in land, can in theory be partially recovered after a long period of time as fossil fuels. Alternatively, virgin biomass, and any waste biomass that results from the processing or consumption of virgin biomass, can be transformed into energy, fuels, or chemicals. The technologies for such conversion include a variety of thermal and thermochemical processes, gasification, liquefaction, and the microbiol conversion of biomass to gaseous and liquid fuels by fermentative methods. Many of these processes are suitable for either direct conversion of biomass or conversion of intermediates. The synthetic fuels produced by these methods are either identical to those obtained from fossil feedstocks, or if not identical, at least suitable as fossil fuel substitutes.
One example of biomass conversion technology are techniques whereby biomass is gasified by partial oxidation to yield a low-calorific-value fuel gas, or synthesis gas, which may then be used as a feed stock in chemical synthesis processes, or as an energy source, for example, to drive an internal combustion engine, a gas turbine or a fuel cell to generate electric power. Such schemes rely on exposing the organic feed stocks to heat and a limited amount of oxygen in specially configured gasifiers to effect partial oxidation of the organic materials, thereby producing an effluent gas consisting primarily of hydrogen and carbon monoxide. Currently, hundreds of companies throughout the world offer such systems for the production of such fuel gas. It is important to note that such methods are effective in converting virtually all organic feed stocks, including biomass, fossil-based organic materials, and their derivatives, including waste derived from the production and use of biomass and fossil-based organic materials, into electrical power.
In addition to the production of synthesis gas through partial oxidation in gassifiers, synthesis gas has also been produced using systems which convert water and organic materials into synthesis gas in a steam reforming reaction. Examples of some such systems include that described in Production of Technological Gas for Synthesis of Ammonia and Methanol from Hydrocarbon Gases, Chemistry, A. G. Leibysh, Moscow, 1971. This paper describes the conversion of methane by steam without catalyst at a variety of different temperatures and different ratios of H2O:CH4. This paper, the entire contents of which are incorporated herein by reference, shows synthesis gas production in both pilot plant experiments and lab results obtained from a quartz reactor. The general trend towards complete conversion of the organic feedstocks into synthesis gas with increasing residence time and temperature is shown in both a graphical presentation and in tables of the observed experimental data. Related experimental work in the United States was reported in “Synthesis Gas Production from Organic Wastes by Pyrolysis/Steam Reforming” Energy from Biomass and Wastes: 1978 Update, by Dr. Michael J. Antal, Jr., the entire contents of which are incorporated by reference. In this work, steam gasification of biomass is accomplished as a two step process. At a relatively low temperature (300° to 500° C.) the biomass is pyrolyzed, producing volatile matter and char. At somewhat higher temperatures (˜600° C.) the volatile matter is then reacted with steam to produce a hydrocarbon rich synthesis gas. The Handbook of Thermodynamic Temperature Process Data, by A. L Suns, 1985, the entire contents of which are incorporated herein by reference, shows the theoretical products of the non-combustive decomposition of methane with water (CH4+2H2O) across increasing temperatures. At 1000° C., the destruction of methane is greater than 99%, and at 1400° C., the destruction of methane is greater than 99.99%.
While these and other gasification systems have shown a wide variety of benefits, several drawbacks are still present in their operation. For example, these types of systems typically are not well suited to processing heterogeneous feed stocks, which are defined herein as feed stocks containing mixtures of organic and inorganic materials. In many cases, the inorganic constituents of the feed stocks can adversely effect the processing of the organic portion, resulting in less than complete conversion, or low processing rates. Also, the inorganic constituents may be left in a highly concentrated ash form, rendering them highly soluble into the environment, particularly ground water, and therefore potentially environmentally hazardous and/or requiring expensive treatment to stabilize these constituents prior to final disposal. Even when operated with homogeneous organic feedstocks, gasification systems typically have drawbacks. For example, a common tradeoff in the operation of a gasification system is between having a clean gas product and minimizing the residual organic product which must be discarded. Typically, a high quality gas is not formed if the organic feedstock is completely gasified. Instead, various oils, tars and other undesired components are present in the gas. Alternatively, a high quality gas may be formed, but only by having less than complete gasification of the feedstock. This results in a waste product of partially oxidized organic material that must be disposed of, often at great cost.
A desire to destroy hazardous organic waste streams has led to their use as a feedstock for steam reforming systems. For example, U.S. Pat. No. 4,874,587 to Terry R. Galloway, the entire contents of which are incorporated herein by reference, describes a system whereby organic liquids are first volatilized into a gaseous form. The volatilized liquids are then mixed with an amount of water in excess of stoichoimetery in the form of steam. This organic gas mixture is then introduced into a first reaction zone maintained at a temperature between 200 and 1400° C. Within this first reaction zone, the steam and organic gas mixture are directed through a “labryinthine path” which presents “organically adsorbent surfaces” to the gaseous mixture. Within this first reaction zone, the labryinthine path and adsorbent surfaces are “selected to provide sufficient temperature, turbulent mixing, and residence time in the first reaction zone for substantially all of the gaseous organic compounds to react with the water.” “Substantially all” of the organic compounds is defined as in excess of 99% and preferably in excess of 99.99% reacted. The gaseous mixture is then passed into a second reaction zone having a temperature range higher than the first and between about 750 and 1820° C. As was the case with the first reaction zone, in the second reaction zone, the amount of water is controlled so that it is equal to or in excess of stoichiometry. The specification states that “the higher temperature of the second reaction zone, together with the lower level of organic compounds entering the second reaction zone, assure that total and complete reaction of the organic compounds results to a level of at least 99.99% and typically much higher.” [sic] The heating for the first and second reaction zones is provided by a plurality of elongated U-shaped hairpin loops of electrical resistance heating elements located within the interior of the second reaction zone.
Systems such as that described by Galloway seek to provide a dual benefit; the destruction of the hazardous organic materials and the creation of a useful synthesis gas. Similarly, waste destruction systems such as that described by Galloway seek to address concerns related to the production of so-called products of incomplete combustion, or PICs, such as hydrofurans and dioxins. To avoid the production of PICs, these types of systems may be operated in reducing environments where conditions for the production of PICs are not favored. In these systems, the energy required to drive the endothermic steam reforming reactions must be provided from a source external to the reaction. Since the energy consumption of these external sources offsets the economic benefit of the synthesis gas produced, the efficiency of delivering this energy is invariably an important consideration in the design of these systems. For this reason, the volatilization of the organic feedstock with a first heating source, prior to steam reforming the resultant gas with a second heating source, as described in both the Antal system and the Galloway system, imposes a significant economic penalty on these systems.
Interest has been directed to the use of plasmas in these steam reforming systems. For example, a process similar to the Galloway system is described in Hydrogen Production by the Hüls Plasma-Reforming Process, G. Kaske, et al., Advanced Hydrogen Energy, Vol. 5, (1986), the entire contents of which are incorporated herein by this reference. In the Kaske system, a plasma is used to reform “gaseous hydrocarbons are reformed with gaseous oxidizing agents, such as steam or carbon dioxide.” The systems described by Kaske thus suffer from many of the drawbacks found in the Galloway systems, in particular, the limitations which arise due to the focus on steam reforming volatilized organic gasses, as opposed to solid or liquid feedstocks.
Plasmas are high temperature, ionized gasses which provide rapid and efficient heat transfer. The ability of plasmas to rapidly transfer heat to incoming organic feedstocks allows the plasma to simultaneously pyrolize the organic feedstocks and provide the thermal energy to drive the endothermic steam reforming reactions of the pyrolyzed organic feedstocks. This dual benefit has been deployed with great success in systems utilizing plasmas including those described in U.S. Pat. No. 5,666,891, titled “Arc Plasma-Melter Electro Conversion System for Waste Treatment and Resource Recovery” to Titus et al. and which the entire contents are incorporated herein by reference, and which shows a variety of particularly useful configurations wherein arc electrodes which produce the plasma are used in systems in various combinations with joule electrodes. In these arrangements, organic compounds contained in the waste are destroyed by pyrolysis, caused by the high temperatures of the plasma breaking the chemical bonds of the organic molecules. By introducing steam into the process chamber, these pyrolyzed organic constituents are converted into synthesis gas, a clean burning fuel consisting primarily of CO, CO2 and H2, through the steam reforming reaction. Other constituents of the waste, which are able to withstand the high temperatures without becoming volatilized, are made to form into a molten state which then cools to form a stable glass. By carefully controlling the vitrification process, the resulting vitrified glass may be made to exhibit great stability against chemical and environmental attack, with a high resistance to leaching of the hazardous components bound up within the glass. In this manner, vitrification may be utilized to convert waste materials into a high quality synthesis gas and a stable, environmentally benign, glass.
While systems utilizing plasma present significant advantages over prior art steam reforming systems, there still exists a need to minimize the energy consumption and capital cost of these systems to increase their economic attractiveness. In particular, the energy required to effect a phase change to form the steam injected in these systems can increase the costs of operating these systems significantly. Thus, there exists a need for more efficient and improved methods of producing synthesis gas from organic and heterogeneous feedstocks.